This invention is directed to imaging techniques. More particularly, the invention provides a method and system for capturing images on CMOS color sensors. Merely by way of example, the invention has been used to capture true color information on each pixel of an N-type substrate CMOS sensor. But it would be recognized that the invention has a much broader range of applicability.
To capture vivid imageries have been long been an endeavor that persisted as long as the human race itself. As early as the Stone Age, people tried to capture of what they see with cave drawings. Over the thousands of years, artists developed techniques for capture images with paint brushes and canvases. With oil paintings, artists have been able to capture real world images with accuracy, but the fidelity of paintings is no match to photography.
In 1826, a French inventor Nicéphore Niépce produced the first photographic image on polished pewter plate covered with a petroleum derivative. Since then, the technique for photographic imaging has been evolved. Better techniques and equipment have improved image quality over the next hundred of years. Over the last fifty years, techniques for color photography have been improved and consummated. In the last decade, with the introduction of the first commercially available digital camera by Kodak in 1990, a new type of image capturing technique, digital imaging, has rapidly become a popular way for capturing images.
For digital imaging, the image sensor (or digital equivalent of film negatives) is one of the most important components for digital imaging devices, such as digital cameras, camera phones, etc. For a long time, image sensors have been based on charge-coupled device (CCD) technology that has been developed by George Smith and Willard Boyle at Bell Labs. In the past, imaging devices have been predominantly CCD based. Recently, CMOS image sensors have become more popular.
The CMOS image sensor technology typically includes millions of sensor pixels (light sensing units), each of the sensor pixel includes two to four transistors and a photodiode. Typically, conventional techniques for CMOS image sensing use one np junction, with very shallow p+ layer applied on top of the N-type region to reduce noise and to enhance the blue color response in image capturing process. In a way, a CMOS sensor unit works in a similar manner as a capacitor. The more charge stored in the electrode, the higher the voltage drop across the depletion region of the CMOS. Light, which is an energy source, generates free carriers. The free carriers under an electric field run towards the N-type region of the CMOS sensor and neutralize the charge and reduce the voltage potential. The voltage difference before and after the integration of energy provides a signal level. The signal level is then used as a reading for the amount of light being detected and used for forming an image.
Depending upon applications, CMOS sensors often have advantages over CCD sensors. For example, compared to CCD image sensors, CMOS image sensors usually provide lower costs and longer battery life. As a result, CMOS image sensors are preferably used in portable imaging devices such as camera phones and point and shoot cameras. At the high end, CCD image sensors are often behind CMOS images sensors in terms of noise level and sensitivity. Because of the many advantages of CMOS image sensors, more resources have been invested in the development of technologies for CMOS image sensors. The resolution of CMOS image sensor has been increased as pixel size shrinks with the MOS transistor channel length. While the increased resolution of image sensors often provides a higher image resolution, the decreased pixel size and the associated increase in noise level have degraded image quality. Various techniques, such as improvements on structure and circuitry, have been developed to improve image quality. For example, various systems and methods have been developed to provide color separation. In general, three major approaches have been used to provide color separation: color filter, stack junction photo diode, and junction separation. Unfortunately, the above-mentioned techniques for image sensing and color separation are often inadequate. These and other limitations of the conventional techniques have been overcome by the present invention.
Therefore, it is desirable to have an improved method and system for a CMOS image sensing device.